1. Field of the Invention
The invention relates to a current detecting apparatus which can precisely detect a current in a range from a small current to a large current and to a control method for such an apparatus.
2. Description of the Related Arts
Hitherto, as a method of detecting a current, an example of detecting a current by using two power sources having a potential difference has been disclosed in JP-A-10-28338 by the same applicant of the present invention. According to JP-A-10-28338, the method is intended to detect a value of a current at the time of charging a secondary battery. When the method is explained with reference to FIG. 19, a voltage E1 of a power source 201 and a voltage E2 of a power source 203 are set so that a potential difference is equal to xcex94V. It is now assumed that the power source 201 can output a low voltage and a large current and the power source 203 can output a high voltage and a small current. A current is detected from a voltage across a resistor 202 having a small resistance value. A current is detected from a voltage across a resistor 204 having a large resistance value.
However, in the example of FIG. 19, there is a problem such that when the small current and the large current are detected, they exercise an influence on each other.
It is, therefore, an object of the invention to provide a current detecting apparatus in which since a detecting range of a current can be enlarged and the current can be independently detected, its influence can be minimized, errors can be reduced, and the loss can be decreased and to provide a control method of such an apparatus.
According to the first aspect of the invention, there is provided a current detecting apparatus comprising: a first resistor provided between an input terminal and an output terminal; first switching means provided in parallel with the first resistor; control means for controlling the switching means; first detecting means for detecting a current flowing in the first resistor or a potential difference which is caused in the first resistor; a second resistor provided so as to be serial to at least the first switching means; and second detecting means for detecting a current flowing in the second resistor or a potential difference which is caused in the second resistor, wherein the control means turns off the switching means when the current or potential difference that is equal to or larger than a first predetermined value is detected by the first detecting means and turns on the switching means when the current or potential difference that is equal to or smaller than a second predetermined value is detected by the second detecting means, and the first and second detecting means outputs the detected current or potential difference. According to the second aspect of the invention, there is provided a control method for a current detecting apparatus comprising: a first resistor provided between an input terminal and an output terminal; switching means provided in parallel with the first resistor; control means for controlling the switching means; first detecting means for detecting a current flowing in the first resistor or a potential difference which is caused in the first resistor; a second resistor provided so as to be serial to at least the switching means; and second detecting means for detecting a current flowing in the second resistor or a potential difference which is caused in the second resistor, wherein when the current or potential difference that is equal to or larger than a first predetermined value is detected by the first detecting means, the switching means is turned off, when the current or potential difference that is equal to or smaller than a second predetermined value is detected by the second detecting means, the switching means is turned on, and the first and second detecting means outputs the detected current or potential difference.
Owing to the switching means provided in parallel with the first registor arranged between the input terminal and the output terminal, the current or potential difference that is obtained from the first resistor and the current or potential difference that is obtained from the second resistor can be detected so that the first and second resistors do not influence each other.
The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.